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3.1) How do I collimate my Dobsonian telescope?
3.2) Should I use a laser collimator? What about Discovery's laser collimator?
3.3) Should I spot my secondary?
3.1) How do I collimate my Dobsonian telescope?
Rather than re-inventing the wheel, I am going to refer you to a selection of very good guides designed to explain the process. But know this -- it's not hard. It's essentially a process of check & tweak -- repeated until you are satisfied with the results. I tell you this because for a long time I avoided buying a Dobsonian scope because I didn't want the "hassle" of collimation. In truth, it only takes a couple minutes each time you set up. Piece of cake. And as with many things, the best way to learn is to have someone show you. Lacking that...
Spend a couple minutes sifting through the guides below and pick one that speaks to your soul, print it out, take the papers to your telescope, and follow along.
- Nils Olof Carlin's Newtonian Telescope Collimation FAQ
- Mel Bartels' Collimating Newtonian Optics
- Skywatcher Telescope's Collimating a Newtonian. Note: This is a PDF file, and requires that you have Adobe Acrobat Reader software.
- Adler Planetarium's Collimating a Newtonian Reflecting Telescope
3.2) Should I use a laser collimator? What about Discovery's laser collimator?
Another big topic on the list. Of particular concern is the quality of the Discovery dot laser collimator. While some people on the list seem to have satisfactory results with it, many people indicate that the units they have are poor in quality. These people advise purchasing other collimation tools.
To educate yourself in the world of laser collimators, hie thee hither to Ed Ting's comparative review of dot and hologram units.
To test your dot collimator (Discovery or otherwise), put in the focuser and twist it around, like you're screwing in a light bulb. While twisting, look at the dot projected onto the primary mirror. If it moves around, it is out of alignment. Some movement is probably inevitable, given the play between the collimator and the focuser. How much is too much? Depends on your taste. If you seek exacting collimation (which, presumably, is the point of the exercise), you may wish to consider only one or two dot diameters an acceptable amount of wiggle. Some people allow more, others even less. My Discovery collimator's dot had about a half inch of wiggle -- far beyond my personal tolerance of error.
Like so much in our hobby, collimation exactitude seems to be a matter of taste, budget, and nit-picketiness. You know how anal retentive you are, you know your budget, and you know your taste. The best possible advise is to find a local club and see what tools the experienced people are using. At a star party, explain that you're looking for collimation tool help. At a sufficiently large gathering, you'll probably end up with a crowd of people at your telescope and more tools in your hand and advice in your ear than you know what to do with. This hands-on approach is invaluable.
Finally, some people have acceptable results using conventional collimation tools - or even film canisters with pin-prick holes! These tools were used for decades, and thousands of amateur astronomers still use them, and wouldn't consider using one of those new-fangled lasers. I've heard one local tell me, "Lasers? Pfaw! I'll only part with my Cheshire eyepiece when you pry it from my cold, dead hands." And so it goes. Tom Hole compares numerous devices here.
I'll cast my lot with the Cheshire folks, after seeing scopes that appeared to be collimated with rather expensive lasers but were clearly way out of whack on star images. Nils Olof Carlin's article in the January 2003 Sky and Telescope explains how this happens and introduces a method of collimating with a Barlowed laser. A simple, free explanation is here. Gene Baraff has a nice folder on the skyquest-telescopes about this (you have to join the group to view). A laser IS handy for checking secondary tilt and focuser wobble. --Wes Stone
3.3) Should I spot my secondary?
[Rich Dodson] Some people strongly advise against it because some (indirect) light rays do strike the center of the secondary, and a spot there might cause some loss of contrast. Others say that the effect of a small spot (~1mm or so in diameter) would be negligible. I have found that it is very easy to align the secondary with a sight tube and/or (especially) a Cheshire tube without a spot, so I finally decided not to spot my secondary. On the other hand, someone said that Takahashi spots the secondaries of their reflectors at the factory, so one would think that there couldn't be too much harm in it. On the other hand, these are short focal length reflectors that are probably used more for photography than for planetary viewing.
My impression is that the warnings about spotting the exact center of the secondary come not from any fear of the spot causing loss of contrast, but because the geometrical center of the secondary is not quite at the optical center of the secondary as it is oriented in the scope. In other words, spotting the secondary could lead to miscollimation. I think this is covered in at least one of the links at the beginning of this page.--Wes Stone
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